On the scale of things

When someone tells me that a thing is so many microns or parsecs across, I
am sometimes somewhat at a loss to picture the scale they're referring to, or
to relate it to the scales of things familiar to me from other discourses. So
here's some information on the scale of things, specifically to make it easier
to compare and contrast between different fields of experience. I am probably
wrong about some of this – please feel free to correct me !

Each page deals with a particular kind of quantity, measured in its unit
within the Système Internationale (SI). I group
things by SI quantifier on the standard unit, putting any thing into the group
for which its value lies between about 0.1 and about 100, with some
arbitrariness near the boundary.

Sources

Where practical, I've simply linked to the source for data in relevant
pages: but books aren't so amenable to HREFs and there are some sites I've
plundered so heavily that it makes sense to mention the site itself here, as
distinct from the various links into it where referenced.

Strays

I don't yet have pages for all types of quantity – most of them would
be rather short on illuminating examples. So I collect strays here; if I get a
lot, it might become another page.

Area density

a human eye's retina has about a fifth of a million
photoreceptors per square
millimetre; that of a
hawk has about a million per square millimetre.

Power

tropical
huricanes dissipate
energy at of order 50 to 200 terawatts. Humanity does so at closer to ten
terawatts.

20115-112-26, LIGO observes grav-waves; two black holes, 14.2 +7.5 solar
masses merge to a 20.8 solar mass result in "a couple of seconds"; solar mass
per second power conversion, to G-radiant. This is 1e4 times as bright as a
GRB. For contrast: our sun has burnt O(1e-6) of its mass in 5e9 years.

See also

For visualising large numbers, you might
find the MegaPenny
project helpful. Lengths, masses and energies are all included in
a spiral scale of
which the internet has propagated some rather crude copies.

Randall Monroe (author of xkcd)
occasionally makes charts to help visualise the scales of things: